This application claims the benefit of the filing date of Taiwan Application Ser. No. 095141606, filed on Nov. 10, 2005, the content of which is incorporated herein by reference.
1. Field of the Invention
The invention relates to a rail-to-rail class-AB operational amplifier.
2. Related Art
FIG. 1 shows a conventional rail-to-rail class-AB operational amplifier. As shown in FIG. 1, the operational amplifier includes a differential pair composed of NMOS transistors N1 and N2, and a differential pair composed of PMOS transistors P1 and P2, and the two differential pairs are connected in parallel to serve as an input. The output currents of the two differential pairs are summed up by a summing circuit composed of transistors N5, N6, N7, N8, P5 and P6, and the summed current is outputted from the node A. Thereafter, the voltage at the node A drives a class-AB output stage composed of transistors N9, N10, N12, N13, N14, P10, P11 and P12 to serve as an output of the operational amplifier so that the very great ability of outputting and sinking a current source would be obtained.
The rail-to-rail class-AB operational amplifier has a relatively large gain and cannot be easily compensated. Usually, a larger compensation capacitor is required and the number of current branches is very great. For example, the PMOS differential pair needs a current consumption branch and the NMOS differential pair needs a current consumption branch. The summing circuit for receiving the outputs of the two differential pairs and generating the voltage outputted form the node A needs to consume two current branches of the transistors N6 and N8. The output stage also needs to consume three circuit branches of the transistors P10, P11 and P12. Thus, the single operational amplifier needs to consume seven current consumption branches. So, the consumed current of the rail-to-rail class-AB operational amplifier is very high and the amplifier is not suitable for the circuit with the low power consumption.
FIG. 2 shows a representative drawing of a combination driver-summing circuit for a rail-to-rail differential amplifier, as disclosed in U.S. Pat. No. 5,311,145. As shown in FIG. 2, the rail-to-rail class-AB operational amplifier includes a differential pair composed of NMOS transistors QI3 and QI4 and a differential pair composed of PMOS transistors QI1 and QI2 to serve as an input. The output currents of the two differential pairs are summed up by a summing circuit, which is composed of transistors QS1, QS2, QS3, QS4, QS5, QS6, QS7 and QS8, and the summed current is outputted to a Class-AB control architecture circuit (transistors QD1 and QD2) to drive an output stage composed of transistors QO1 and QO2 to serve as an output of the operational amplifier so that the good ability of outputting and sinking a current source can be obtained.
In this rail-to-rail class-AB operational amplifier, the class-AB control architecture circuit and the summing circuit have been combined together to reduce the number of the power consumption branches. The operational amplifier exclusive of the current control circuits of the input stage transistors QI5 and QI6, the bias voltage generating circuits of the transistors QD1 and QD2, and the transistors QD3, QD4, QD5, QD6, QD7, QD8 and IDB leaves the main circuit of the operational amplifier. The power-consumption branches of the main circuit include one current branch composed of the transistors QI1 and QI2, one current consumption branch composed of the transistors QI3 and QI4, two current branches required by the summing circuit and including one current branch Is and the other current branch composed of the transistors QD1 and QD2, and a current consumption branch composed of the output transistors QO1 and QO2. So, the main circuit of the operational amplifier needs five current consumption branches, which is lower than that of the first prior art by two branches. However, the rail-to-rail class-AB operational amplifier cannot sufficiently save the power, and needs more transistors and the larger area in the prior art.